1. Field of the Invention
The present invention relates to a twisted nematic (TN) liquid crystal display, and more particularly, it relates to a TN liquid crystal display which includes pixels arranged in a matrix and uses a non-linear active element as a switching element for each pixel.
2. Description of the Related Art
Liquid crystal displays have many applications. They are used not only as relatively low information content displays in calculators and digital watches, but also as high information content displays in word processors, personal computers, and the like.
As a driving method for such liquid crystal displays, a duty driving method and a static driving method are known. Recently, an active driving method has often been used. In the active driving method, pixels are respectively connected to active elements so that the on/off state of each pixel is controlled by means of a corresponding active element. A three-terminal element such as a thin film transistor (TFT) and a two-terminal element such as a diode are known as typical active elements. The active driving method is used in a twisted nematic (TN) liquid crystal display.
FIG. 3A is a schematic diagram showing the construction of a conventional TN liquid crystal display employing such an active driving method. In the liquid crystal display of FIG. 3A, polarizing plates 1 and 3 are provided on both sides of a liquid crystal cell 2, respectively. The liquid crystal display is so positioned that the polarizing plate 1 is located on the front side. The liquid crystal cell 2 includes a pair of transparent substrates 4 and 4', and a nematic liquid crystal layer 5 sandwiched therebetween. A pair of alignment films (not shown) are formed on the surfaces of the substrates 4 and 4' facing the liquid crystal layer 5, respectively. The substrates 4 and 4' are arranged so that the liquid crystal molecules of the liquid crystal layer 5 make a 90.degree. twist going from the substrate 4' to the substrate 4.
In the optical design for an active matrix type liquid crystal display such as shown in FIG. 3A, it is necessary to consider the relationship between the capacitance and resistance of an active element and those of the liquid crystal layer 5, and also to consider the anisotropy of refractive index (.DELTA.n) of the liquid crystal material used for the liquid crystal layer 5. The following describes an optical design for the liquid crystal display of FIG. 3A in which, for example, an MIM (Metal-Insulator-Metal) element, one of the two-terminal non-linear elements, is used as the active element.
FIG. 4 shows an equivalent circuit of a single pixel portion of such a liquid crystal display employing an MIM element. In order to attain high resolution and small crosstalk in the liquid crystal display employing the MIM element, it is desirable that the ratio of the capacitance C.sub.L of the liquid crystal to the capacitance C.sub.M of the active element (MIM element) shown in FIG. 4, i.e., C.sub.L /C.sub.M, be large. It is known that the MIM element exhibits excellent characteristics when C.sub.L /C.sub.M is equal to or Greater than 10. The MIM element has, for example, a Ta/Ta.sub.2 O.sub.5 /Ti structure. In this case, when the Ta.sub.2 O.sub.5 insulating film of the MIM element has a dielectric constant .epsilon..sub.r of 24 and a thickness of 700 .ANG., and the size of the MIM element is 5 .mu.m.times.5 .mu.m, the element capacitance C.sub.M becomes 0.076 pF. Accordingly, in order to obtain a capacitance ratio C.sub.L /C.sub.M of 10 or more, the liquid crystal capacitance C.sub.L is required to be equal to or greater than 0.76 pF. In order to obtain a liquid crystal capacitance C.sub.L of 0.76 pF in a TN liquid crystal display having pixels with a dot size of, for example, 0.25 to 0.3 mm, the dielectric constant .epsilon..sub.r of the liquid crystal material in the range of 3 to 14 and the cell gap d of the liquid crystal cell in the range of 4 .mu.m to 10 .mu.m are required, in view of practical use.
In general, a fluorinated liquid crystal is used as the liquid crystal material for an active matrix type liquid crystal display. This is because the liquid crystal material is required to have high resistivity so as to improve the characteristics of the active element. The fluorinated liquid crystal has a low dielectric constant, and accordingly, when it is used for a liquid crystal cell, the cell gap d thereof is required to be small in order to obtain the above-described value of the liquid crystal capacitance C.sub.L. The fluorinated liquid crystal also has a small value of .DELTA.n (the anisotropy of refractive index). Thus, when such a fluorinated liquid crystal is used to prepare a liquid crystal cell having a capacitance ratio C.sub.L /C.sub.M of 10 or more, the resultant liquid crystal cell is of a first minimum design and usually used in the first minimum condition.
A liquid crystal display using such a liquid crystal cell of a first minimum design is disclosed in Japanese Patent Publication No. 4-14329. In this liquid crystal display, a liquid crystal cell having a .DELTA.n.multidot.d value of 210 to 600 nm is used in the normally white mode (the positive transmission or reflection mode), and good display quality is attained.
FIGS. 3B and 3C show the relationship between the transmission axes 101a and 103a of the polarizing plates 1 and 3 and the rubbing directions 104 and 104' of the substrates 4 and 4' of the liquid crystal cell 2 in the normally white mode. The rubbing direction of the substrate is an orientation direction of the liquid crystal molecules which are in contact with the substrate. In the normally white mode, the first polarizing plate 1 and the second polarizing plate 3 are so positioned that their transmission axes 101a and 103a intersect each other at right angles. For example, it is herein assumed that the liquid crystal display uses a 90.degree. TN liquid crystal cell having a cell gap d of 4.5 .mu.m with the anisotropy of refractive index (.DELTA.n) being 0.087. On one side of this liquid crystal cell 2 of a first minimum design, the polarizing plate NPF-F1205Du (manufactured by Nitto Denko Corporation) is provided as the first polarizing plate 1, in such a manner that its transmission axis 101a is parallel to the rubbing direction 104 of the adjacent substrate 4. On the other side of the liquid crystal cell 2, the polarizing plate NPF-F3205M (Nitto Denko Corporation) is provided as the second polarizing plate 3, in such a manner that its transmission axis 103a is parallel to the rubbing direction 104' of the adjacent substrate 4'. With this arrangement, the liquid crystal display can attain relatively good display quality.
The above-described liquid crystal cell of the first minimum design, however, cannot attain good display quality when it is used in a normally black mode liquid crystal display where the polarizing plates 1 and 3 are so positioned that their transmission axes 101a and 103a are parallel to each other. FIGS. 3D and 3E show examples of the arrangement of the polarizing plates 1 and 3 in the normally black mode liquid crystal display. In such a liquid crystal display, the liquid crystal molecules in the vicinity of the substrates 4 and 4' of the liquid crystal cell 2 cannot move easily due to a strong wall effect. Accordingly, when light passes through the liquid crystal cell 2, the behavior of the light in the vicinity of the substrates 4 and 4' is different from that in the bulk of the liquid crystal. As a result, in the liquid crystal cell 2 of the first minimum design having a small .DELTA.n.multidot.d value, light leakage arises, so that neither the display background nor the display image obtained by the application of an OFF voltage can be made to have an excellent tone of black. For example, when one or two kinds of polarizing plates selected from among NPF-G1220DuN, G1225DuN, G1220Du, G1225Du, F1220Du and F1225Du (all manufactured by Nitto Denko F1220Du and F1225Du (all manufactured by Nitto Denko Corporation) are used as the two polarizing plates 1 and 3 disposed respectively on both sides of the liquid crystal cell 2 of the first minimum design, the background color becomes bluish black instead of pure black. The display color obtained by the application of an OFF voltage also becomes bluish black. The display color obtained by the application of an ON voltage becomes bluish white.
If a liquid crystal material having a larger dielectric constant is used or the cell gap d is made larger, it is possible to solve the above-mentioned problem of the low-quality black display in the liquid crystal cell of the first minimum design used in the normally black mode. But these changes in the liquid crystal cell design will change the ratio (C.sub.L /C.sub.M) of the liquid crystal capacitance C.sub.L to the element capacitance C.sub.M. The change in C.sub.L /C.sub.M decreases resolution and increases crosstalk, thereby causing significant deterioration of display quality.